Method for producing reduced iron agglomerate

ABSTRACT

Provided is a method for producing a reduced iron agglomerate, which comprises introducing an agglomerate comprising an iron-oxide-containing substance, a carbonaceous reducing agent, a melting point adjuster and an aid for the melting point adjuster onto a furnace bed of a moving-bed-type heating furnace and then heating the agglomerate on the furnace bed to reduce iron oxide in the agglomerate; and melting the resultant product by additional heating to cause the coalescence of an iron component, thereby producing the reduced iron agglomerate, wherein an agglomerate containing the melting point adjuster that has an average particle diameter of 0.3 mm or less and in which the content of a particle diameter of 0.5 mm or less in an amount of 55 mass % or more is used. Thus, provided is a method which can improve the yield of a reduced iron agglomerate having a large particle diameter and can also reduce the amount of time required for the production of the reduced iron agglomerate to improve the productivity of the reduced iron agglomerate.

TECHNICAL FIELD

The present invention relates to a method for producing a reduced iron agglomerate. The method includes introducing an agglomerate made from a mixture containing an iron oxide-containing substance and a carbonaceous reducing agent onto a hearth of a moving-bed heating furnace and heating the agglomerate to reduce and melt iron oxide in the agglomerate.

BACKGROUND ART

The following method has been developed: a direct reduction ironmaking method for obtaining massive (including granular) metallic iron (reduced iron) from a mixture containing an iron oxide source (hereinafter referred to as “iron oxide-containing substance” in some cases) such as iron ore or iron oxide and a carbon-containing reducing agent (hereinafter referred to as “carbonaceous reducing agent” in some cases). In the ironmaking method, massive metallic iron (reduced iron agglomerate) is obtained in such a manner that an agglomerate formed from the mixture is introduced onto a hearth of a moving-bed heating furnace and is heated by gas heat transfer or radiation heat in the heating furnace using a heating burner, iron oxide in the agglomerate is thereby reduced with the carbonaceous reducing agent, and obtained reduced iron is subsequently carburized, is melted, is agglomerated while being separated from co-produced slag, and is then solidified by cooling.

The ironmaking method does not require a large facility such as a blast furnace and is highly flexible in terms of sources because of no need of coke. Therefore, in recent years, the ironmaking method has been increasingly investigated for practical use. However, for industrial implementation, the ironmaking method needs to be further improved in terms of operation stability, safety, economical efficiency, granular iron (product) quality, productivity, and the like.

In particular, in the production of reduced iron agglomerates, it is desired that the yield of a reduced iron agglomerate with a large particle diameter is increased and the production time thereof is reduced. Regarding such a technique, for example, Patent Literature 1 proposes “a method for producing granular metal, comprising heating a source material containing a metal oxide-containing substance and a carbonaceous reducing agent to reduce a metal oxide in the source material, heating a produced metal to melt the metal, and agglomerating the melted metal while the melted metal is separated from co-produced slag, wherein an agglomeration promoter for co-produced slag is blended in the source material”.

In this technique, the agglomeration promoter (for example, fluorite) is blended. Therefore, it can be expected that a granular metal with a large particle diameter can be produced in relatively high yield. However, in this technique, an improvement effect is saturated; hence, a further increase in effect is desired.

CITATION LIST Patent Literature

PTL 1: Japanese Unexamined Patent Application Publication No. 2003-73722

SUMMARY OF INVENTION Technical Problem

The present invention has been made in view of the above circumstances. It is an object of the present invention to provide a production method which increases productivity by increasing the yield of a reduced iron agglomerate with a large particle diameter and by reducing the production time thereof in the case where the reduced iron agglomerate is produced in such a manner that an agglomerate made from a mixture containing at least an iron oxide-containing substance and a carbonaceous reducing agent is heated in a moving-bed heating furnace and iron oxide in the agglomerate is reduced and is melted.

Solution to Problem

A method for producing a reduced iron agglomerate according to the present invention is capable of solving the above problem and is summarized as follows: the method includes introducing an agglomerate containing an iron oxide-containing substance, a carbonaceous reducing agent, a melting-point adjuster, and an aid for the melting-point adjuster onto a hearth of a moving-bed heating furnace; heating the agglomerate to reduce iron oxide in the agglomerate; and melting the resultant product by further heating to agglomerate an iron component, wherein an agglomerate containing the melting-point adjuster which has an average particle diameter of 0.3 mm or less and in which the content of particles with a diameter of 0.5 mm or less is 55% by mass or more is used.

In the method according to the present invention, the melting-point adjuster (one acting directly on a gangue component) is particularly at least one of dolomite and limestone. The aid (one promoting the reaction of the melting-point adjuster) for the melting-point adjuster is particularly fluorite (a calcium fluoride-containing substance).

In the method, the melting-point adjuster is present in a central portion of the agglomerate and it is preferred that the average particle diameter of the melting-point adjuster is 0.3 mm or less and the content of the particles with a diameter of 0.5 mm or less in the melting-point adjuster is 55% by mass or more. In this case, the average particle diameter of the melting-point adjuster, which is present in the central portion, may be appropriately adjusted depending on the type of the melting-point adjuster.

Another method, capable of solving the above problem, according to the present invention is a method for producing a reduced iron agglomerate. This method includes introducing an agglomerate containing an iron oxide-containing substance, a carbonaceous reducing agent, a melting-point adjuster, and an aid for the melting-point adjuster onto a hearth of a moving-bed heating furnace; heating the agglomerate to reduce iron oxide in the agglomerate; and melting the resultant product by further heating to agglomerate an iron component, wherein an agglomerate containing the aid for the melting-point adjuster is used, the aid having an average particle diameter of 90 μm or less, the content of particles with a diameter of 50 μm or less in the aid being 35% by mass or more.

In this method also, the melting-point adjuster is particularly at least one of dolomite and limestone. The aid for the melting-point adjuster is particularly fluorite (a calcium fluoride-containing substance).

Advantageous Effects of Invention

According to the present invention, a reduced iron agglomerate is produced in such a manner that an agglomerate made from a mixture comprising at least an iron oxide-containing substance, a carbonaceous reducing agent, a melting-point adjuster, and an aid for the melting-point adjuster is introduced onto a hearth of a moving-bed heating furnace; the agglomerate is heated such that iron oxide in the agglomerate is reduced; and the resultant product is melted by further heating such that an iron component is agglomerated. The yield of a reduced iron agglomerate with a large particle diameter is increased and the production time thereof is reduced in such a manner that the average particle diameter of the melting-point adjuster is reduced and the content of particles with a predetermined diameter in the melting-point adjuster is appropriately controlled or in such a manner that the average particle diameter of the aid for the melting-point adjuster is reduced and the content of particles with a predetermined diameter in the aid is appropriately controlled. This enables productivity to be increased.

DESCRIPTION OF EMBODIMENTS

In order to form an agglomerate from a mixture containing an iron oxide-containing substance, a carbonaceous reducing agent, a melting-point adjuster, and an aid for the melting-point adjuster, the iron oxide-containing substance, the carbonaceous reducing agent, the melting-point adjuster, and the aid being raw material components (hereinafter referred to as “components”) used to produce a reduced iron agglomerate, both the melting-point adjuster and the aid for the melting-point adjuster are adequately pulverized into particles with an adequate size. However, the influence of the size (average particle diameter) of these components on the yield or productivity of the reduced iron agglomerate has not been taken into account. It has been believed that pulverizing these components into excessively fine particles causes the dispersion of these components, prevents the agglomeration of reduced iron, and therefore reduces productivity.

In order to achieve the above object, the inventors have performed investigations from various angles. In particular, the inventors have investigated the influence of the average particle diameter or particle size distribution (the content of particles with a predetermined diameter) of these components on the yield and productivity of the reduced iron agglomerate. As a result, the inventors have found that the above object is achieved well in such a manner that the average particle diameter of the melting-point adjuster or the aid for the melting-point adjuster is reduced and the content of particles with a predetermined diameter is appropriately adjusted, thereby completing the present invention.

In the present invention, it is necessary that the average particle diameter of the melting-point adjuster contained in the agglomerate is 0.3 mm or less and the content (the proportion to the whole melting-point adjuster) of particles with a diameter of 0.5 mm or less in the melting-point adjuster is 55% by mass or more. Alternatively, it is necessary that the average particle diameter of the aid for the melting-point adjuster contained in the agglomerate is 90 μm or less and the content (the proportion to the whole aid for the melting-point adjuster) of particles with a diameter of 50 μm or less in the aid is 35% by mass or more. Incidentally, the term “average particle diameter” as used herein refers to the particle diameter (hereinafter referred to as “D50” in some cases) where the number of particles corresponds to 50% by mass (the cumulative value is 50% by mass) as counted from the smallest particle size. The use of the fined melting-point adjuster or the fined aid for the melting-point adjuster to form the agglomerate increases the yield and productivity of the obtained reduced iron agglomerate. The reason for this can be inferred as described below.

The agglomerate is reduced and melted at a high temperature of 1,200° C. to 1,500° C. In the initial stage of the reduction reaction, reaction proceeds through the direct contact of the iron oxide-containing substance with the carbonaceous reducing agent. Pulverizing the melting-point adjuster, such as limestone or dolomite ore, or the aid, such as fluorite, for the melting-point adjuster into fine particles reduces the distance between a gangue component contained in the iron oxide-containing substance and the surface of the melting-point adjuster or the aid for the melting-point adjuster (increases the probability of the presence of the gangue component near the surface of the melting-point adjuster or the aid for the melting-point adjuster) and is unlikely to prevent the agglomeration of the reduced iron agglomerate (hereinafter referred to as “granular reduced iron” in some cases) because the gangue component is likely to come into contact with the melting-point adjuster or the aid for the melting-point adjuster to produce a melt. That is, it is conceivable that a phenomenon completely opposite to a conventionally recognized finding may occur.

In order to effectively exhibit such an effect, the melting-point adjuster which has an average particle diameter of 0.3 mm or less and in which the content of the particles with a diameter of 0.5 mm or less is 55% by mass or more needs to be used to form the agglomerate. Alternatively, the aid for the melting-point adjuster needs to be used to form the agglomerate, the aid having an average particle diameter of 90 μm or less, the content of the particles with a diameter of 50 μm or less being 35% by mass or more. Incidentally, the content of the particles with a diameter of 0.5 mm or less in the melting-point adjuster is preferably 60% by mass or more and more preferably 65% by mass or more (may be 100% by mass). The content of the particles with a diameter of 50 μm or less in the aid for the melting-point adjuster is preferably 40% by mass or more and more preferably 45% by mass or more (may be 100% by mass).

In the present invention, the iron oxide-containing substance may be iron ore, iron sand, nonferrous refining residue, or the like. The carbonaceous reducing agent may be, for example, a carbon-containing substance such as coal or coke.

The agglomerate may contain another component such as a binder, a MgO-containing substance, or a CaO-supplying substance. The binder may be, for example, a polysaccharide (for example, starch such as cornstarch, rice flour, or wheat flour). The MgO-containing substance may be, for example, a MgO powder, natural ore, a Mg-containing substance extracted from seawater, magnesium carbonate (MgCO₃), or the like. The CaO-supplying substance may be, for example, quicklime (CaO) or the like.

The shape of the agglomerate is not particularly limited and may be, for example, a pellet shape, a briquette shape, or the like. The size of the agglomerate is not particularly limited. The diameter (maximum diameter) of the agglomerate is preferably 50 mm or less. Excessively increasing the diameter of the agglomerate impairs the efficiency of granulation and also impairs the transfer of heat to a lower portion of a pellet to reduce productivity. Incidentally, the lower limit of the diameter of the agglomerate is about 5 mm.

The whole of the melting-point adjuster in the agglomerate need not be fined. A portion (for example, 10% by mass or more) of the melting-point adjuster to be used may meet specified requirements (the average particle diameter is 0.3 mm or less and the content of the particles with a diameter of 0.5 mm or less is 55% by mass or more). A form meeting such requirements is, for example, the presence of the fined melting-point adjuster only in at least a central portion of the agglomerate. That is, when the agglomerate is heated from outside, the increase in temperature of the central portion of the agglomerate is slow as compared to the periphery thereof and the reaction of the central portion is also slow. In order to mitigate this phenomenon, fining the melting-point adjuster present in the central portion of the agglomerate is effective. Incidentally, the term “central portion” refers to a portion ranging from the center of a sphere to a location satisfying the above content (a portion outside the location is referred to as “peripheral portion”) when the agglomerate has, for example, a spherical shape (a dry pellet described below).

When the fined melting-point adjuster is present in at least the central portion of the agglomerate, it is a basic form that the fined melting point adjuster is present only in the central portion of the agglomerate as specified in the present invention and the melting-point adjuster with a usual average particle diameter (not fined) is present in a peripheral portion. The case where the whole of the melting-point adjuster used meets the requirements specified in the present invention is included in embodiments of the present invention.

The present invention is further described below in detail with reference to examples. The examples are not intended to limit the present invention. Appropriate changes may be made within the scope adaptable to the gist described above and below and are included in the technical scope of the present invention.

This application claims benefits of priority to Japanese Patent Application No. 2013-039421 filed on Feb. 28, 2013. The entire contents of Japanese Patent Application No. 2013-039421 filed on Feb. 28, 2013 are incorporated herein by reference.

EXAMPLES Example 1

An agglomerate was prepared from a mixture containing an iron oxide-containing substance, a carbonaceous reducing agent, melting-point adjusters, an aid for the melting-point adjusters, and a binder. The agglomerate was introduced onto a hearth of a moving-bed heating furnace and was heated. Iron oxide in the agglomerate was reduced and was melted, whereby a reduced iron agglomerate (granular reduced iron) was produced.

The iron oxide-containing substance used was iron ore having a composition shown in Table 1 below. The carbonaceous reducing agent used was coal having a composition shown in Table 2 below. The melting-point adjusters used were limestone having a composition shown in Table 3 below and dolomite having a composition shown in Table 4 below. The aid for the melting-point adjusters used was fluorite having a composition shown in Table 5 below. Agglomerates were produced by varying the average particle diameter (D50) and particle size distribution (the content of particles with a predetermined diameter) of each of the melting-point adjusters and the aid for the melting-point adjusters (Table 7 below). In particular, wheat flour used as the binder was mixed with each of mixtures containing the melting-point adjusters (limestone and dolomite) and the aid (fluorite) for the melting-point adjusters, the melting-point adjusters and the aid being different in average particle diameter and particle size distribution, at a blending ratio shown in Table 6 below. An adequate amount of water was added to each of the mixtures, followed by producing raw pellets with a diameter of 19 mm using a tire-type pelletizer. The obtained raw pellets were introduced into a dryer and were heated at 180° C. for 1 hour such that contained water was completely removed, whereby pellet-shaped agglomerates (spherical dry pellets) were prepared.

TABLE 1 Composition of iron ore (mass percent) T•Fe FeO SiO₂ CaO Al₂O₃ MgO S 66.62 0.12 2.24 0.07 0.96 0.03 0.008

TABLE 2 Composition of coal (mass percent) Fixed carbon Volatile matter Ash Total 79.5 15.97 4.53 100.00

TABLE 3 Composition of limestone (mass percent) SiO₂ CaO Al₂O₃ MgO S 0.16 55.59 0.22 0.26 <0.001

TABLE 4 Composition of dolomite (mass percent) SiO₂ CaO Al₂O₃ MgO S 0.84 30.0 0.28 20.44 0.062

TABLE 5 Composition of fluorite (mass percent) SiO₂ T.Ca Al₂O₃ MgO F 10.58 44.01 1.36 0.1 43.63

TABLE 6 Blending ratio (mass percent) Iron ore Coal Limestone Dolomite Fluorite Binder Total 75.05 18.0 2.9 2.35 0.8 0.9 100.00

The dry pellets were introduced into a moving-bed reduction heating furnace in which a carbon material (anthracite with a maximum particle diameter of 2 mm or less) was spread and were heated at 1,450° C. in a nitrogen atmosphere, followed by measuring the time (reaction time) necessary for reduction and melting.

The results are shown in Table 7 together with the average particle diameter and particle size distribution of components (iron ore, coal, limestone, dolomite, and fluorite) used (regarding iron ore, the average particle diameter thereof only is shown and the same applies hereinafter). Common properties (such as apparent density and dry pellet analytical data) of the dry pellets are also shown in Table 7. A measurement method and standards for main items among items shown in Table 7 are as described below.

(Productivity (Productivity Index))

The productivity where granular reduced iron is produced in such a manner that the dry pellets are heated and iron oxide is reduced and is melted is evaluated through the production (tons) of granular reduced iron per hearth area (m²) per unit time (hour) as represented by the following Equation (1):

Productivity (tons/m²/hour)=productivity of granular reduced iron (tons/hour)/hearth area (m²)tm (1).

In Equation (1), the production (tons/hour) of granular reduced iron is represented by the following Equation (2):

Productivity of granular reduced iron (tons-granular reduced iron/hour)=amount of introduced agglomerates (dry pellets) (tons-agglomerates/hour)×mass of granular reduced iron produced from a ton of agglomerates (tons-granular reduced iron/ton-agglomerates)×product recovery rate  (2).

In Equation (2), the product recovery rate is calculated as the mass ratio [mass % of +3.35 mm granular iron/amount % of granular reduced iron×100 (%)] of granular reduced iron with a diameter of 3.35 mm or more to the amount of obtained granular reduced iron (shown as “Yield of +3.35 mm granular iron (%)” in Table 7). In Table 7, in order to quantitatively evaluate effects of the present invention, agglomerates (dry pellets) of Experiment No. 1 are used as standard agglomerates and the productivity where other agglomerates are used is expressed as a relative value (productivity index) on the basis that the productivity where the standard agglomerates are used is 1.00.

TABLE 7 Experiment No. 1 2 3 4 5 Average particle diameter (D50) Iron ore (μm) 37 37 37 37 37 Coal (μm) 21 21 21 21 21 Limestone (μm) 1102 276 1102 1102 1102 Dolomite (μm) 1155 1155 204 99 1155 Fluorite (μm) 103 103 103 103 85 Content of −500 μm in limestone (mass percent) 22 59 22 22 22 Content of −500 μm in dolomite (mass percent) 20 20 63 81 20 Content of −50 μm in fluorite (mass percent) 34 34 34 34 38 Dry pellets Apparent density (g/cm³) 2.181 2.313 2.287 2.272 2.275 Reaction time (minutes) 11.15 10.56 11.00 10.24 10.88 Analytical data of dry pellets Total iron (%) 51.11 51.39 51.24 50.73 51.16 Granular reduced iron Yield of +3.35 mm granular iron (%) 85.73 92.4 90.97 94.83 89.84 Analytical data of granular reduced iron C (%) 3.10 2.98 2.90 3.09 2.97 Productivity index (—) 1.00 1.21 1.13 1.25 1.12

As is clear from the results, it is understandable that when the average particle diameter (D50) of limestone used as a melting-point adjuster is 0.3 mm or less (300 μm or less) and the content of particles with a diameter of 0.5 mm or less (shown as “−500 μm”) is 55% by mass or more (Experiment No. 2) or when the average particle diameter (D50) of dolomite used as a melting-point adjuster is 0.3 mm or less (300 μm or less) and the content of particles with a diameter of 0.5 mm or less (shown as “-500 μm”) is 55% by mass or more (Experiment Nos. 3 and 4), the yield of granular reduced iron is increased and the productivity thereof is significantly increased. On the other hand, it is understandable that when the average particle diameter (D50) of fluorite used as an aid for a melting-point adjuster is 90 μm or less and the content of particles with a diameter of 50 μm or less (shown as “−50 μm”) is 35% by mass or more (Experiment No. 5), the yield of granular reduced iron is increased and the productivity thereof is significantly increased.

Example 2

Dry pellets having a double structure were prepared using a mixture (the blending ratio was the same as that shown in Table 6) containing an iron oxide-containing substance, carbonaceous reducing agent, melting-point adjusters (limestone and dolomite), aid for the melting-point adjusters (fluorite), and binder each having the same composition as that used in Example 1. In particular, wheat flour used as a binder was mixed with a mixture containing iron ore, limestone, and fluorite each having an average particle diameter and particle size distribution shown in the column “central portion” in Table 8 below and an adequate amount of water was added to this mixture, followed by producing spherical raw pellets with a diameter of 9.5 mm using a tire-type pelletizer. A mixture containing limestone having different average particle diameters and particle size distributions was concentrically formed on the periphery (a peripheral portion) of each spherical raw pellet using the spherical raw pellet as a core, whereby raw pellets with a diameter of 19.0 mm were produced (the mixture content of a central portion was about 12% by mass of each pellet). The obtained raw pellets were introduced into a dryer and were heated at 180° C. for 1 hour such that contained water was completely removed, whereby pellet-shaped agglomerates (double-structured pellets) were prepared.

The double-structured pellets were introduced into a moving-bed reduction heating furnace in which a carbon material (anthracite with a maximum particle diameter of 2 mm or less) was spread and were heated at 1,450° C. in a nitrogen atmosphere, followed by evaluating the time (reaction time) necessary for reduction and melting in the same manner as that described in Example 1. The results are shown in Table 8 together with the average particle diameter (D50) and particle size distribution of components (iron ore, coal, limestone, dolomite, and fluorite) used. Incidentally, items evaluated in Examples 1 and 2 are also shown in Table 8 (an evaluation method is the same as that described in Example 1).

TABLE 8 Experiment No. 6 Position Peripheral Central portion portion Average particle diameter (D50) Iron ore (μm) 37 37 Coal (μm) 21 21 Limestone (μm) 276 1102 Dolomite (μm) 1155 1155 Fluorite (μm) 103 103 Content of −500 μm in limestone 59 22 (mass percent) Content of −500 μm in dolomite 20 20 (mass percent) Content of −50 μm in fluorite 34 34 (mass percent) Dry pellets Apparent density (g/cm³) 2.275 Reaction time (minutes) 10.76 Analytical data of dry pellets Total iron (%) 51.55 Granular reduced iron Yield of +3.35 mm granular iron (%) 93.53 Analytical data of granular reduced iron C (%) 2.91 Productivity index (—) 1.19

As is clear from the results, it is understandable that the effect of increasing the yield of granular reduced iron is achieved in such a manner that the whole of each pellet is not fined but the central portion only is primarily fined, that is, an effect of the present invention is achieved even in such a state that the amount of a fined component is small.

INDUSTRIAL APPLICABILITY

The present invention provides a method for producing a reduced iron agglomerate. The method includes introducing an agglomerate containing an iron oxide-containing substance, a carbonaceous reducing agent, a melting-point adjuster, and an aid for the melting-point adjuster onto a hearth of a moving-bed heating furnace; heating the agglomerate to reduce iron oxide in the agglomerate; and melting the resultant product by further heating to agglomerate an iron component, wherein an agglomerate containing the melting-point adjuster which has an average particle diameter of 0.3 mm or less and in which the content of particles with a diameter of 0.5 mm or less is 55% by mass or more is used. This enables the following method to be provided: a method which increases productivity by increasing the yield of a reduced iron agglomerate with a large particle diameter and by reducing the production time thereof. 

1. A method for producing a reduced iron agglomerate, comprising: introducing an agglomerate comprising an iron oxide-containing substance, a carbonaceous reducing agent, a melting-point adjuster, and an aid for the melting-point adjuster onto a hearth of a moving-bed heating furnace; heating the agglomerate to reduce the iron oxide-containing substance; and melting the agglomerate by further heating the agglomerate to coalesce an iron component, wherein the melting-point adjuster has; an average particle diameter of 0.3 mm or less, and a content of particles having a diameter of 0.5 mm or less of 55% by mass or more, based on a total mass of the melting-point adjuster.
 2. The method according to claim 1, wherein the melting-point adjuster is at least one of dolomite and limestone.
 3. The method according to claim 1, wherein the aid is fluorite.
 4. The method according to claim 1, wherein the melting-point adjuster is present in a central portion of the agglomerate.
 5. A method for producing a reduced iron agglomerate, comprising introducing an agglomerate comprising an iron oxide-containing substance, a carbonaceous reducing agent, a melting-point adjuster, and an aid for the melting-point adjuster onto a hearth of a moving-bed heating furnace; heating the agglomerate to reduce the iron oxide-containing substance; and melting the agglomerate by further heating the agglomerate to coalesce an iron component, wherein the aid has: an average particle diameter of 90 μm or less, and a content of particles having a diameter of 50 μm or less of 35% by mass or more, based on a total mass of the aid.
 6. The method according to claim 5, wherein the melting-point adjuster is at least one of dolomite and limestone.
 7. The method according to claim 5, wherein the aid is fluorite. 